One of fields of Nicola Tesla experiments was wireless energy transfer. Since many years ago it has been implemented to create a very simple AM radio receiver which needs no power source. It gets all the power it needs straight from the RF signal and can drive a small speaker or earphones.

Some years ago this technology was used in RFID chips, very small radio tranceivers which can be used as a replacement for barcodes.

But in CES 2007 some months ago, Powercast presented an innovative dime-size receiver which can convert RF signals to DC power. The receiver has to be no longer than 1 meter (3 feet) away from the power transmitter and can supply devices with up to 6 Volts power (no Watts info, read the EDIT). The innovation here is that the Powercast's solution can use energy from signals bouncing off the walls of a room, which have slightly different frequency than the original source, achieving up to 70% efficiency! 

Don't you think we should see this embedded in a future version of Zune which can sync and charge without cables ?

EDIT: Some more technical details: Powercast uses 905.8 MHz frequency (WPT9066 module) and is optimized for frequency range at 902-928 MHz, but other bands can be used also. For consumer electronics, a 2 Watt implementation is considered safe when the distance from the transmitter is more than 10 inches (25 cm) for 30 minutes continuous exposure. 

Read more: Powercast, CNN Money, Engadget, C|Net (Video Review), Business 2.0, EE Times